1. Field of the Invention
The present invention relates to apparatus for indicating the angle of a drafting parallel and, more particularly, to apparatus for use with a drafting board and a drafting parallel of the type including a straight edge suspended by cables on one surface of the board for parallel movement therealong for setting and indicating the angle of the straight edge.
2. Description of the Prior Art
In the field of drafting, there are three major types of apparatus for drawing parallel lines on a drafting board. The first type consists of a simple T-square. The second type consists of a drafting machine including a series of arms and linkages fixedly connected to the drafting board, at one end thereof, and movable over the face of the board. The third type is referred to as a drafting parallel and includes a straight edge suspended by cables on one surface of the drafting board for parallel movement therealong.
The most popular of the three types is the drafting parallel since it has several advantages. One is that it can be made with a length to extend across the entire width of the drafting board. Secondly, the angle of the straight edge may be adjusted over a wide range and this angle will remain fixed as the straight edge moves along the surface of the drafting board.
However, a problem has existed in setting the angle of the straight edge. The most commonly used technique is to use an adjustable triangle, lining one edge of the triangle up with a known reference line on the drafting board and then moving the straight edge until it is parallel to another edge of the triangle. While this appears satisfactory, it is not, for at least two reasons. First of all, existing adjustable triangles are somewhat confusing and until one becomes very familiar therewith, it is often difficult to remember which sides of the triangle are used for setting the angle of the straight edge. Secondly, the scales on typical adjustable triangles are very small and it is difficult to accurately set angles within 1/2.degree..